1. The Field of the Invention
This invention broadly relates to the preparation of phenol-aldehyde foam and the foamed products thus prepared. In some of its more specific aspects, the present invention is concerned with the preparation of phenol-aldehyde foam by a method employing a novel frothing technique followed by acid curing of the froth under conditions which produce a closed cell structure in the final solid foamed product. The invention further provides the improved closed cell phenol-aldehyde foam prepared by the method of the invention.
2. The Prior Art
Phenol-aldehyde foam has been prepared for many years by a method which involves the addition of an acid catalyst to a liquid phenol-aldehyde resole resin. Polymerization of the resole resin commences immediately following the addition of the acid catalyst and, inasmuch as the reaction is highly exothermic, the liberated heat rapidly increases the temperature of the reaction mixture. This continuing increase in the reaction temperature progressively increases the rate of polymerization and the reaction proceeds to completion very rapidly. In fact, once initiated, the reaction is generally considered to be impossible to control. The temperature of the reaction mixture increases sufficiently to cause the formation of enough steam from the water originally present in the resole resin and the water of reaction to foam the resin, and the accompanying high reaction temperature causes the initially liquid resole resin to cure to the solid infusible state before the foam collapses. The phenol-aldehyde foam thus prepared has an open cell structure and does not have optimum properties for use as thermal insulation or as a moisture barrier. The foam is also friable and has insufficient strength, flexibility and/or abrasion resistance for use by the construction industry or in engineering applications in general. Open cell phenol-aldehyde foam has a tendency to punk, but in general it has excellent fire resistance and generates only a small amount of smoke when subjected to high temperatures. These latter two properties could be used to great advantage in thermal insulation applications provided the phenol-aldehyde foam also has a high closed cell content and adequate strength.
In recent years, a number of methods have been disclosed in the prior art which are said to produce closed cell phenolic foam. However, such prior art methods have not been commercially successful when using unmodified phenol-formaldehyde resole resins. In all instances, these prior art methods produce foam from phenol-formaldehyde resole resins which have one or more of the deficiencies mentioned hereinbefore, and/or require the use of expensive modifying additives or impractical processing techniques, and/or are not capable of consistently producing high quality closed cell phenol-formaldehyde foam on a commercial scale and on a reproducible basis.
The deficiencies and disadvantages mentioned above have prevented the phenol-aldehyde foams commercially available heretofore from successfully competing with polyurethane foam as thermal insulation in spite of their markedly superior fire resistance and low smoke generation properties. Polyurethane foam has a number of important advantages thereover, including excellent physical and mechanical properties and a high closed cell content, and it may be easily prepared by commercially available methods. These and other properties and advantages have caused polyurethane foam to be used extensively as thermal insulation by the construction and other industries, and in numerous other applications. However, the polyurethane foams are not fire resistant, and they produce a dense and toxic smoke when heated to the point of char. This problem is so pronounced that the use of polyurethane foams as thermal insulation in the construction industry is being strongly discouraged at the present time for safety reasons.
It is apparent from the foregoing that those skilled in the art have long sought an entirely satisfactory and commercially acceptable method of preparing high quality closed cell foam from liquid phenol-aldehyde resole resins on a predictable and reproducible basis. However, such a method was not available prior to the present invention in spite of the great need therefor.